The present invention relates to microwave devices generally and more specifically to matched impedance waveguide termination devices used for absorbing high microwave power propagated down a waveguide transmission line.
In high power microwave applications it is often necessary to terminate a transmission line with a substantially matched load capable of absorbing and dissipating the power transmitted into the load. Methods of terminating a waveguide transmission line have been developed involving solid materials as the power absorbing medium, however, in most cases the absorbing medium is water. Where water is used the general class of termination devices is generically referred to as "water loads".
An object in designing a water load is usually to produce a load with high power handling capability, low power reflection (i.e. low VSWR characteristics), relative broadband frequency operation, and relative simplicity of manufacture. These objectives should be achieved for applications involving high pulse and average power, typically pulse power levels in the range of megawatts to tens of megawatts or higher over the bandwidth desired, and average power levels as high as kilowatts to hundreds of kilowatts over the bandwidth desired.
It is also desirable for certain applications to provide a water load which maintains its performance for all orientations of the guide and for modest changes in fluid temperature or in the temperature of the surrounding environment. This means that the fluid flow characteristics should be maintained and entrapment of air bubbles prevented for all guide orientations; otherwise there will be a deleterious effect upon the amount of power reflected by the termination. In conventional water loads, maintaining uniform flow characteristics, as well as providing for an initially low reflected power, is a particular problem which the present invention seeks to overcome.
Conventionally, water loads have been constructed with a fluid carrying dielectric plug which is tapered or bullet shaped and which is positioned to extend down the center of the waveguide where the point of the plug receives the incident power in a region of high electric field. The mismatch at the end of the plug in this high electric field region causes undesirable reflections. One attempted solution to the mismatch problem has been to simply move the fluid carrying plug off center relative to the center axis of the guide. For example, a rectangular waveguide water load has been constructed having a straight sided plug with a pencil point end where the plug is placed against one of the sidewalls of the guide away from the highest on-axis E field concentrations. While some improvement in the mismatch characteristics of the load has been achieved by placing the dielectric plug off center in this manner, the physical dimensions of the plug have always presented enough of an abrupt physical transistion to the E fields in the transmission line to cause undesirable reflections.
It is an object of the present invention to reduce reflected power relative to conventional loads. It is noted that the accepted measure of reflective power in a waveguide is given by the voltage standing wave ratio, commonly denoted VSWR. The present invention seeks to provide a water load having a VSWR value less than 1.2 for the operating frequency range of the load. This corresponds to reflective power levels of less than 0.83 percent. It is also the object of this invention to provide lower VSWR values, less than 1.05, within certain narrow frequency operating ranges.
It is intended that the present invention be particularly suitable for use in connection with applications where the orientation of the power transmission system varies with time as dictated by the system's application. An example of such a system would be medical accelerators of the type rotated about a patient in both a vertical and horizontal plane.
The present invention is also intended to provide a high peak and average power absorbing termination which is mechanically small, and easy to construct and assemble.